Applicant claims priority under 35 U.S.C. xc2xa7119 of German Application No. 199 49 439.8 filed Oct. 14, 1999. Applicant also claims priority under 35 U.S.C. xc2xa7120 of PCT/DE00/03572 filed Oct. 11, 2000. The international application under PCT article 21(2) was not published in English.
1. Field of the Invention
The invention concerns to a procedure for the photometric determination of the quality of gas, particularly of burning gases, according to the precharacterising part of claim 41.
2. The Prior Art
For determining the quality of gas for example in distribution networks for natural gas or the like, devices for measuring the quality of the guided-through gases are used. These devices measure the condition of gas. Natural gas, because of being a natural product according to its origin and by mixture shows respective fluctuations in respect of its composition, whereas the composition for example of natural gas coming from the different hydrocarbons determines the caloric value from extrapolated quantities. Therefore it is of great importance to measure the guided-through amount of gas in a gas supply network and therewith the respective amount of energy, to determine the exact respective condition at the feeding point into the natural gas network and at the deliverance points of the customers and therewith to deduct a definite transported or supplied amount of energy. In doing so for the customer of the gas, an invoice can always state the actual supplied amount of energy relating to different conditions of the gas and a correspondingly varying amount of energy. Vice versa, the detection of the condition of the gas offers the guarantee for the customer to obtain a desired quality and therewith a required amount of energy.
The determination of the quality of gas obtains additional relevance, since with the elimination of the guiding-through monopoly, the suppliers of natural gas use the same network for delivering gases of quite different provenance and therefore also different composition. Only an easy and cost-effective detection of the condition of the gas by means of cost-effective measuring devices and methods allows for a controllable and accurate accounting.
For the measurement of the quality of gas as relevant quantities, the standard volumetric gross calorific value Hv,n, the standard density xcfx81n and the compressibility coefficient K have to be determined as accurately as possible and also regarding the different gas qualities.
In practice, for the settlement of account, the transported volume Vb of the of the gas at working conditions (pressure pb, temperature Tb) is measured by means of flow measuring devices. With knowledge of the condition of the gas, the compressibility coefficient K can be determined, with which the volume of the gas Vn at standard conditions (pressure pn, temperature Tn) is calculated.       V    n    =                              p          b                ⁢                  T          n                                      p          n                ⁢                  T          b                      ⁢          xe2x80x83        ⁢          1      K        ⁢          V      b      
By means of multiplication of this standard volume with the volumetric gross calorific value Hv,n at standard conditions, the transported amount of energy Q can be obtained:
Q=VnHv,n
Alternatively, the volume at working conditions Vb can directly be multiplied with the volumetric gross calorific Hv,b at working conditions (Energymeter).
Another important quantity for applications with natural gas is the thermal output of gas burners; this varies in accordance to the gas quality and is characterized by means of the so-called Wobbe index Wv: gases with the same Wobbe index Wv deliver the same thermal output at a burner nozzle. For calculating the Wobbe index Wv the standard density xcfx81n of the gas is required, from which the relative density according to air is determined (dv=xcfx81gas/xcfx81air)       W    v    =            H      v                      d        v            
Therefore the determination of the gross calorific value Hv,n at standard conditions has central relevance for the practical determination of the quality of gas, for example, for accounting purposes.
Until now different devices for the measurement of the gas quality are used. So-called direct and so-called indirect procedures are known. By using direct procedures, the quantities to be determined are measured separately and therefore the gas is transformed to standard conditions, by which expensive treatments of the gas are required.
The gas condition can be determined most easily by means of so-called calorimeters, in which by means of an open flame a gas probe is burnt and submitted to a cooling medium. Heat quantity and the thereupon detectable temperature rise of the cooling medium the calorific value of the burnt gases can be determined. Such devices will need a complicated mechanic for the adjustment of a certain quantitative proportion of gas, combustion air and for example cooling air as cooling medium and are therefore expensive and error prone, and enhanced security requirements for the devices are necessary due to the open burning. Also the maintenance and calibration have to be carried out by qualified personnel, and the calorimeter must be used in conditioned rooms. Therefore the purchase and operating costs of such test assemblies are very high.
Using calorimetry by means of catalytic burning (for example with pellistors) the probe gas is mixed with air and burnt at the 400 to 500xc2x0 C. hot helixes of a catalyst. The temperature rise of the catalyst is about proportional to calorific value. Because this procedure is based on a sensitive surface effect, it is subject to strong drifts and necessitates frequent calibration with search gas. The catalytic calorimeter is most favorable of all mentioned procedures, however, because it is better suited for control than for accounting because of its accuracy.
The direct measurement of the density xcfx81b at working conditions is done with hydrostatic balances, which are very expensive precision devices, with which the buoyancy of a ball filled with nitrogen is measured in accordance to the density of the surrounding medium, here of the probe gas. With another procedure a thin-walled metal cylinder, which is positioned by a current linkage of the probe gas, is set in oscillation. The density of the surrounding gas determines the resonant frequency of the cylinder, which is captured as a sensitive measured quantity. Both procedures are very expensive for the determination of the standard density, because you they require an adjustment to the standard conditions.
The compressibility coefficient K cannot be measured directly, but instead can be calculated by means of different numerical standard-arithmetic techniques out of the directly measurable gas quantities. One of these procedures, the so-called GERG88-procedure (DVGW-worksheet 486) needs the input quantities listed in table 1 below. The amount of substance of CO2 is determined according to the state of the art by a non dispersive infrared-spectroscopical procedure (NDIR), whereby the gas must be brought into a defined condition near or at standard conditions. The amount of substance of H2 is of significance only when working with coke oven gases and can be left unattended in the typical natural gases today distributed in Europe. The compressibility coefficient K can be determined to 10xe2x88x923 with the help of the GERG88-equation in case of sufficient accuracy of the input quantities.
The other procedure for determination of the behavior of real gases is done according to the AGA8-92DC-equation (ISO 12213-2:1997 (E)). This process requires as input quantities the amount of substance of 21 leading gas components (table 2) and has an accuracy of 10xe2x88x923.
The state of the technology includes, besides the direct measurement techniques, also the indirect measurement of the gas quality by means of gas chromatography. A defined volume of the probe gas is brought into a defined condition and is carried by a carrier gas, typically helium, through a system of gaschromatographic separation columns. Due to their different retention times, the individual gas components reach the downstream sensor, generally this is a detector for caloric conductibility, at the end of the separation column at different times. The peak area of the sensor signal can therefore be interpreted as amount of substance, whereas the evaluation must be carried out in comparison with a reference gas, that must have a similar composition to the probe gas.
The drawback of the gas chromatography is the expensive sample preparation and installation of the whole system, and the expensive maintenance and operation by well-trained personnel. From the amounts of substances of the individual gas components that the gas chromatography delivers, all relevant gas quantities can be calculated. For the implementation of such indirect measurements via chromatography, automatically working process chromatographs with detectors for caloric conductability are deployed. These devices generally measure eleven components of the natural gas (N2, CO2, CH4, C2H6, C3H8, C4H10, C5H12, C6+ and so on) Helium is used as the carrier gas, but its light volatility in practice often leads to prematurely emptying of the bottle for the carrier gas and therefore leads to short maintenance cycles for such a device. A gas is chosen as the calibrating gas that is similar to the natural gas to be measured.
Such chromatographic systems carry out measurement cycles without interruption, in order to capture changes in the quality of the gas immediately. This leads to a high consumption of carrier gas and calibrating gas and requires that maintenance of the device be performed in relatively short intervals.
It is also known to determine the composition of a gas with conventional infra-red-gas-analyzers. Such analyzers working in the middle infra-red or near infra-red area do not offer the requirements of high precision and stability for a determination of the caloric value under the measurement conditions which are required here.
Also, a parallel reference measurement must be carried out beside the intrinsic measurement of the probe gas, for the sake of compensating the at least essential influences of failures. As a measuring result, the known infra-red-gas analyzers deliver superimposed frequency spectrums, that make it very difficult to form a conclusion regarding individual components of a inspected gases.
In the literature there has been described an infrared spectroscopical procedure for gas analysis (Optical BTU sensor Development,xe2x80x9d Gas Research Institute GRI-93/0083), that determines by means of-so-called multivariate analysis (MVA) of the near infrared spectrum of gases the volumetric concentration of the amount of substances of the carbon-containing components of the gas and therewith of the volumetric caloric value under operating conditions. This procedure does not deliver the calorific value Hv,n under standard conditions or the standard reference density xcfx81n and the amount of substance of CO2, so that it is not qualified for the determination of the compressibility coefficient K nor the complete determination of the quality of a gas. Determining the calorific value by means of known photometric methods requires necessary equipment for the realization of the procedures, whereby a benefit in the speed of the coverage of the absorption spectrums of the natural gas in near or middle infrared spectral area is obtained. The entire absorption spectrum of the natural gas is therefore put together from of the sum of single spectrums of components represented in gas and therefore can be measured and analyzed with the aid of more appropriate methods of spectral analysis.
Doing this, the ascertained quota of extinction of a component in the entire spectrum of the natural gas is equivalent to the part of the concentration of this component within search gas (so-called Beer-Lambert-law). With the knowledge of the calorific value of this respective component, the calorific value of the entire mixture of gases can then be calculated as a summation value. This procedure of spectral analysis has the problem, however, of the intense overlap by absorption bands of different components, which frequently lead to inaccurate results and beyond that to a high calculation effort. One more infrared spectroscopical procedure according to DE 198 38 301 A1 acts as a direct spectral evaluation (DSA) with a spectral function, with which the spectrum of the gas is folded. The procedure allows the specification of the volumetric calorific value Hv,b under operating conditions direct from the spectrum. While burning the gas, the respectively caused heat of the reaction is based on the combustion of Cxe2x80x94H-bindings and a thereby caused heat quantity depends to the present binding energy. This is thereby exploited, that the oscillations of the Cxe2x80x94H-bindings, which are equal to each other, have a certain binding energy and produce the same heat quantity during a combustion, interact with an associated wavelength of an electromagnetic radiation. It is possible by means of a wavelengths-resolved measuring and a wavelengths-dependent valence of the grade of interaction of these oscillations to calculate the calorific value Hv,b of the gas, without the requirement of an identification of individual gas components. This document additionally discloses a device for the realisation of such a procedure, which is tuned for the specific requirements of the measuring method and enables a weighted summing up of the grades of interaction. With this procedure, the calorific value Hv,b of a mixture of gases under operating conditions can be determined, the other quantities relating to the determination of gas quality can itself not be determined with measuring techniques.
It is further known a procedure according to DE 199 00 129.4 A1, in which the density of a probe gas at working conditions is determined with an optical-spectroscopical procedure. This procedure proceeds on the assumption, that each binding of the IR-active gas components contributes to the extinction, whereby with each binding the mass of the coupled atoms is associated. In this manner, the mass of all the IR-active atoms is represented by contributions to absorption within the spectrum. The frequency of oscillation of each binding and therewith its spectral location depends on the reduced mass of the partners of binding. The spectrum contains in its amount and its spectral distribution information for determination of the density of the gas. The context between the spectrum and the mass in the measuring volume and therewith with the density at working conditions of the gas is described by a spectral weighting function, which can be called spectral density. The contribution of masses of the IR-inactive components will be calculated via the stated quantities and an appropriate formulation for the compressibility behaviour. In such a procedure, multiple iterations are required, for which certain assumptions with regard to the initial conditions must be taken, which possibly can be problematic with regard to the convergence of the iteration.